1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming system and, more specifically, to a sheet processing apparatus that includes a creasing unit and an image forming system that includes the sheet processing apparatus and an image forming apparatus, such as a copying machine, printer, facsimile, or digital multifunction peripheral that has the functions of a copying machine, printer, and facsimile in combination.
2. Description of the Related Art
Conventionally, center-folding or center-folded bookbinding is performed to a plurality of sheets, recording sheets, transfer sheets, or sheet-like recording members such as film-like members (hereafter, referred to as “sheets” in this specification) discharged from an image forming apparatus in such a way that the discharged sheets are combined together in a sheaf and bound together at the center section thereof, and the sheaf of center-bound sheets is folded in two at the center section. If whole of the sheets in a sheaf are folded together, the folded area of the outer sheet in the sheaf is stretched more than that of the inner sheet. Because an image area, on which the image is formed, on the folded area of the outer sheet is stretched, damage, such as toner coming off, may occur on the image area. The same phenomenon occurs in other folding processes such as Z-folding or tri-folding even when a binding operation is not performed. A sheet in a sheaf may be insufficiently folded due to the thickness of the sheaf.
A creasing device, called a creaser, is already known. Before a folding process, such as a process for folding a sheaf of sheets in two, is performed, the creasing device forms a crease in advance on an area of the sheet that is to be folded so that even the outer sheet can be easily folded, preventing toner from coming off the sheet. In such a creasing device, a crease is formed on a sheet in a direction perpendicular to the conveying direction by using a unit, such as a roller, laser, or creasing blade for pressing.
Japanese Patent Application Laid-open No. 2009-051667 describes the invention of a creasing device. The object of this invention is to improve the shape of a finished sheet by using a simple configuration. The invention is characterized by a creasing device that includes a conveying mechanism that conveys a sheet in a conveying direction; a creasing unit that forms a crease on the sheet; a sensor that detects the tilt of the sheet in the conveying direction; and an aligning mechanism that rotates the creasing unit so as to align the creasing unit with the tilt of the sheet detected by the sensor. In this creasing device, the creasing unit includes a disk-shaped creasing blade that is moved and rotated above the sheet and forms a crease on the sheet.
For a creasing process performed in a conventional manner, a creasing device is used as a single device (as a mechanism); therefore, it is necessary to allow for a space corresponding to the length of a sheet in the conveying direction. Specifically, if a creasing process is performed by a post-processing system, a creasing device is placed between the image forming apparatus and the post-processing (stapling, center-binding, punching, stacking, folding, or the like) apparatus. In this system, a sheet is received by the creasing device, is subjected to a sheet-alignment correction process, such as skew correction, is stopped at a creasing position, is subjected to creasing, and is conveyed downstream in the conveying direction for post-processing. In such a case, because sheet alignment correction and creasing are performed within the creasing device, the creasing device needs a space corresponding to the size of a sheet in the conveying direction, thus the device itself takes a large space. Furthermore, there has been a problem of increase in power consumption because the creasing device needs to have a sheet-alignment correcting mechanism or a position setting mechanism. If different mechanisms are used as a sheet-alignment correcting mechanism and a position setting mechanism in the creasing device and the post-processing apparatus, errors occur during the sheet-alignment correction process and the position setting process in the sheet-alignment correcting mechanism and the position setting mechanism, which results in a problem of uneven accuracy.